I realized that I started to understand something in programming as soon as I got a job and worked for several months, I realized that I understood a lot just wrong, but did not know much at all. Therefore, the phrase: "I have already finished learning PHP at a basic level" is nonsense, implement some interesting project for you, put it on githab, and then again here but with the theme "I have already finished my first project" :)
And what to study further - it depends on what you are interested in, set a goal to do something, think about how you will do on the basis of what and ... go !!!